1. Field of the Invention
The present invention relates to a process for producing mirocapsules which use amino-aldehyde resin as a microcapsule wall and are used for changing or controlling various properties of substances in such a field as pharmaceuticals, agricultural chemicals, perfumes, dyestuffs etc., specifically for producing pressure-sensitive recording sheets.
2. Prior Art
The encapsulating methods can be divided into a mechanical method, physical method, physicochemical method, chemical method, and methods based upon combination of these.
The present invention relates to an in-situ polymerization method in which the capsule-forming reaction starts from the continuous phase. Further, the capsule-forming reaction of the present invention is carried out in the presence of an anionic water-soluble copolymer.
Use of amine/formaldehyde polycondensate or urea/formaldehyde polycondensate for forming the wall film has already been disclosed in, for example, Japanese Patent Publication Nos. 12380/1962, 3495/1969, and 23165/1972. With these methods, it is difficult to efficiently and stably deposit the polycondensate around the hydrophobic core material, or the emulsification or dispersion is not efficiently performed.
In order to improve the above-mentioned defects, Japanese Patent Publication No. 16949/1979 proposes to use, as the anionic high-molecular electrolyte, a polyethylene/maleic anhydride, polyacrylic acid, polymethyl vinyl ether/maleic anhydride, and the like. This method improves the emulsification and dispersion properties and the depositing efficiency of the polycondensate around the core material, but provides as a defect the resulting capsule slurry having an undesirably high viscosity. The combined use of a copolyer of styrene/maleic anhydride and a copolymer of vinyl acetate/maleic anhydride has been disclosed in Japanese Patent Laid-Open No. 47139/1980, the use of an anionic polymer of vinylbenzene sulfonic acid type in Japanese Patent Laid-Open No. 51238/1981, the use of polymer or copolymer containing a sulfo group in Japanese Patent Laid-Open No. 58536/1981.
In the process using melamine-formaldehyde polycondensate or urea-formaldehyde polycondensate as a capsule wall, further, there have been disclosed the use of an anionic water-soluble copolymer containing of at least these monomers including acrylic acid, hydrooxyalkyl acrylate or hydrooxyalkyl methacrylate and styrenesulfonic acid in Japanese Patent Laid-Open No. 14942/1983, the combined use of an acrylic acid copolymer and a styrenesulfonic acid copolymer in Japanese Patent Laid-Open No. 142836/1984, the use of a copolymer of acrylic acid, acrylonitrile, acrylic amide alkyl sulfonate and acrylic acid sulfoalkyl ester in Japanese patent Laid-Open No 28819/1985, and the use of acrylic acid and alkyl acrylate in Japanese Patent Laid-Open No. 58045/1985.
The above-described methods have following defects to be improved. In Japanese Patent Laid-Open No. 47139/1980, the solubility of styrene-maleic anhydride copolyer is insufficient when the pH is smaller than 4; in Japanese Patent Laid-Open No. 51238/1981, the production of a dense capsule wall is difficult, a large quantity of foam is formed, which makes the workability of the capsule production difficult in the use of a vinylbenzene-sulfonic acid-type polymer for preparing a capsule wall film composed of urea-formaldehyde polycondensate.
In Japanese Patent Laid-Open Nos. 14942/1982, 14283/1984 and 68045/1985, it is disclosed that good slurries can be obtained with high concentration, low viscosities and particle size distribution of capsules. However, there is the disadvantage that in their use for pressure-sensitive recording sheets, spots appear on the surface of these sheets. In self-coloring type pressure-sensitive recording paper, the spots appear markedly. This is thought to be due to large particles and coagulated product slightly present in a capsule slurry. If a surfactant is used to remove them, there arises a new problem that the wall film impermeability of a capsule and the coloring property are deteriorated. In the use of the copolymer disclosed in Japanese Patent Laid-Open No 68045/1985, the emulsification property and emulsification stability are improved without fail. However, there is room for improvements with regard to the spots and the viscosity increase of the copolymer itself with the lapse of time.